Abstract: The recent progress in the investigation of structural changes in polymer liquids (i.e. polymer melts and concentrated solutions) by mechanical spectroscopy was reviewed. The effects of molecular weight and composition of polymer blends on the liquidliquid transition were analyzed, and the mechanism of the liquidliquid transition was clarified. The relationships between the phase behaviors of concentrated aqueous solutions of poly (ethylene oxide)poly(propylene oxide)poly(ethylene oxide) (PEOPPOPEO) and temperature, concentration and the addition of NaCl were presented. The outlook on some of the most promising areas that being studied through mechanical spectroscopy in the area of soft matter was also pointed out.